Smart Tradelists automatically add cards to your tradelist when they are in your collection. For example, if you want to trade all cards for a particular player or perhaps there is a set, that you don't care about, you can add that set to your tradelist automatically. You can of course still add cards to your tradelist manually in the collections tab.
For example, if you don't care about keeping your Albert Pujols cards and want to offer them to Pujols collectors as trade bait, you can create a Player Smart Tradelist based on Albert Pujols. This will automatically add all Albert Pujols cards to your tradelist.